Merge github.com:grpc/grpc into slice_with_exec_ctx

examples/python/helloworld/greeter_client.py

examples/python/helloworld/greeter_server.py

examples/python/helloworld/helloworld_pb2.py

examples/python/helloworld/helloworld_pb2_grpc.py

examples/python/multiplex/helloworld_pb2.py

examples/python/multiplex/helloworld_pb2_grpc.py

examples/python/multiplex/multiplex_client.py

examples/python/multiplex/multiplex_server.py

examples/python/multiplex/route_guide_pb2.py

examples/python/multiplex/route_guide_pb2_grpc.py

examples/python/multiplex/run_codegen.py

examples/python/route_guide/route_guide_client.py

examples/python/route_guide/route_guide_pb2.py

examples/python/route_guide/route_guide_pb2_grpc.py

examples/python/route_guide/route_guide_server.py

src/compiler/python_generator.cc

src/core/lib/security/credentials/jwt/jwt_verifier.c

src/core/lib/security/credentials/jwt/jwt_verifier.h

src/core/lib/support/string.c

src/core/lib/support/string.h